A conventional cathode ray tube for television reception is composed of a faceplate, a funnel, a neck and an electron gun. These several components are commonly formed separately, hermetically sealed together, and the assembly evacuated to form a completed tube.
A cathode ray tube, especially a television receiver tube for color reception, operates under a high voltage and emits strong x-radiation. The hazardous nature of this radiation has necessitated the development of glasses capable of absorbing such radiation. Advances in the television art, particularly in the area of color receivers, have led to use of ever increasing voltages in the tube operation. In turn, this has required glasses with increased x-ray absorption capability. The glass neck of a tube, being thinner than the faceplate, must have a much higher x-ray absorption value than is required for a faceplate. Currently absorption values for the neck glass of at least 95 cm.sup.-1 at 0.6 Angstrom units are desired.
A further property of critical concern in a cathode ray tube glass, especially a neck glass, is electrical resistivity. Values of log R at 350.degree. C. of at least 8.0 ohm-cm are considered desirable. To achieve such values, control of alkali metal oxides is exercised. Thus, lithia (Li.sub.2 O) is customarily excluded from a glass and soda (Na.sub.2 O) is minimized.
The neck portion of a cathode ray tube is customarily drawn from a glass melt as tubing. It is conventional practice to draw such tubing by a Vello or a downdraw process. The tubing is drawn continuously from the forehearth and bowl of a continuous glass tank. Such a process is depicted and described, for example, in U.S. Pat. Nos. 2,009,326 and 2,009,793 (Vello) and at pages 210-211 of the glass text, Glass: The Miracle Maker. C. J. Phillips, Pitman Publishing Corporation (1941).
An unwanted phenomenon of critical importance in drawing glass tubing is devitrification, that is, a tendency for the glass to crystallize, at the tube drawing temperature. A glass can be kept adequately hot in the tank to avoid devitrification. Once on the draw, tubing cools sufficiently rapidly that devitrification is not a problem. The critical area is the forehearth/bowl area as the glass exits to the draw.
Commercial glasses, with otherwise excellent properties, have been found to have a tendency to devitrify in the critical forehearth/bowl area. This may lead to unacceptable crystal inclusions in the glass tubing. More troublesome is a tendency for crystal buildup to occur in the forming region, that is, around the bowl opening. This causes distortion in the tubing shape and complete loss of product.
It then becomes necessary to shut down the draw and apply excess heat to the bowl to melt and flush out the crystal deposits. Such remedial action is not only costly and time-consuming, but is technically unsatisfactory as well. Sufficiently high temperatures to do a thorough cleansing job can not be employed without endangering the bowl structure.
Accordingly, an intensive compositional search has been undertaken to find a glass family having more favorable internal liquidus characteristics. At the same time, of course, other required properties, in particular, x-ray absorption and electrical resistivity values, had to be retained. It is the basic purpose of my present invention to provide a narrow composition family of lead silicate glasses that meet the need.